The invention relates to a process for forming an initial section or like component featuring a hollow interior to a final shape by means of high internal pressure in the sealed hollow interior using a medium that can flow, in particular forming until the final section comes into contact with the wall of a shape-determining space. Further, the invention also relates to a section with a space delimited by section walls, in which two section walls each determine a corner region of the cross-section, in particular an initial section for carrying out this process.
In the high-internal-pressure-forming (HIPF) process a hollow section is expanded by means of internal pressure. In addition, by means of at least one stem engaging the part in question, the hollow section may be displaced and widened, compressed or expanded.
German Patent DE 35 32 499 C1 describes a device for hydraulic expansion of a length of pipe by using a plug-like cylindrical probe which can be introduced into the pipe and, using at least a pair of sealing rings spaced a distance apart, forms a circular space which is filled with compressive medium for the purpose of expanding the tube. Each of the sealing rings is situated in a ring-shaped grooved with a u-shaped in cross-section, in the probe and initially upon introducing the probe into the tube, has an outer diameter which at most is the same as the outer diameter of the probe. Before starting the expansion process, in order to seal the ring-shaped gap between the probe and the tube, compressive medium is introduced into the ring-shaped grooves via a feed pipe connected to the medium supply line and applies compressive force radially to the sealing rings. The feeding of the compressive medium to the ring-shaped space is performed solely by way of at least one of the grooves and is controlled by a sealing ring acting as a valve, which closes off an opening between the groove and the ring-shaped space until it has achieved its sealing function by elastic expansion. That groove is provided with at least one inclined slit at its edge neighboring the ring-shaped space. If the pressure in the ring-shaped space between the two seals is increased, the wall of the tube begins to expand in this region.
This internal high pressure forming or hydroforming process is finding ever increasing application in the automobile industry as an economical means for manufacturing car body components. Mainly steel tubes are employed as starting material. The final contour of the component to be shaped this way is generally much more complicated than the simple circular cross-section of the starting material. As a rule, the HIPF process results in regions which are much more heavily deformed than other regions and which are correspondingly thinner. If these regions are subjected to a high degree of loading in use, the initial sheet must be sufficiently thick; this however results in an unnecessary amount of material in the less heavily formed regions. This disadvantage is contrary to the requirement of obtaining the lowest possible weight in the component.
Recently aluminium alloys have been included along with steel as starting material for HIPF processes. As with steel there are manufacturing processes in which tubes of aluminium sheet are employed as starting material; alternatively, extruded aluminium sections may also be employed for that purpose. For economic reasons extruded steel sections do not come into question here. The use of extruded sections has the decisive advantage that the shape of the initial section is almost without limit.
HIPF processes using extruded sections are employed mainly to be able to produce high precision parts. To that end the present state of the art tends to make the shape of the initial section as close as possible to that of the final section in order to employ relatively small degrees of deformation in the HIPF-process. In particular with curved components that are to be bent in advance or where the section cross-sections feature sharp corners, this approach is usually not successful. Also, attempts to keep the degree of deformation small generally results in its non-uniform distribution. As a resultxe2x80x94and due to the pre-shaping from the bending processxe2x80x94spring-back effects are produced causing the desired precision to be achieved only in exceptional cases using that process. Likewise as a rule, sharp corners which exhibit a large ratio of wall thickness to outer radius cannot be filled out using this process.
In HIPF-processes using steel pipes it is normal to carry out pre-shaping prior to the actual shaping process (bending and HIPF)xe2x80x94this e.g. in order to arrive at a more favourable cross-section for bending or in order to make it even possible to place the part in the HIPF shaping tool.
In view of the above, the object of the present invention is to provide a specific cross-section of an extruded section which achieves a favorable distribution of deformation in the HIPF-process; the elastic spring-back of the component after removal from the HIPF shaping tool should be minimized and dimensional accuracy achieved to the desired degree of precision.
In accordance with the present invention, in order to shape-form the initial section featuring at least one corner region, the wall sections adjacent to the corner region are pre-shaped in a curved mannerxe2x80x94as viewed in cross-sectionxe2x80x94counter to the direction of applied pressure, and subsequently reshaped by applying the high internal pressure of the medium that can flow in the direction in which the pressure is applied, displacing the corner region. If there are at least two corner regions present, the wall lengths between the corner regions are accordingly pre-shaped counter to the direction in which the pressure is applied and reshapedxe2x80x94likewise by applying the high internal pressure of the medium that can flow, displacing the corner regions in the direction in which the pressure is applied.
In practice the reshaping will mainly concern angles that are almost right angles, whereby the section cross-sections need not have rectangular shaped contours. However, other sizes of angle can be reshaped, in particular corners running to a peak with angles of less than 45xc2x0.
It has been found favorable to carry out the displacement of the corner region in the direction of the line bisecting the angle or its line of symmetry. In the initial section this corner region should also be of greater thickness.
The local degree of deformation can be created in the initial section in the form of oversizing with respect to the final contour of the final section, this by means of a domingxe2x80x94inward pointing curvature in the section cross-section. It is also possible to introduce the degree of deformation in the initial section in the form of undersizing with respect to the final contour of the final section.
Usefully, therefore, the requirements for precise light weight construction are met i.e. the initial section is designed in such a manner that at the end of the HIPF process the component exhibits an accumulation of material mainly in those places where, for reason of strength, this is required. In order to achieve the above mentioned goals:
the local degree of deformation of the section wall is controlled by curvature in the cross-section and by lengths of section with local undersizing and, in this connection, the internal stress oriented in the longitudinal direction;
section corners are made more pointed;
those lengths of section which should undergo little or no deformation are made thicker;
section cross-sections are curved in advance.
Controlling the local degree of deformation by means of dome-like, inwards pointing curvature of the cross-section, and section lengths that are undersized locally, is achieved using the following principle.
The inwards pointing curvature of the cross-section is important here; especially with regard to cross-sections whose section walls are curved in the final component, it is emphasized that it depends on the relative curvature and not on the absolute curvature. This is so because in the end this determines whether the contour of the initial sectionxe2x80x94with respect to the final contourxe2x80x94exhibits oversizing or undersizing, through which the behavior of the component in the described shape-forming process is controlled.
By doming or similar cross-sectional curvature it is possible to achieve local oversizing. In contrast to domed oversizing on the outside of the section, this doming does not cause any problem on placing the component into the mold or on closing the mold. In the HIPF process the oversizing causes local compression of the material in the direction along the periphery of the section. As a result of the constant volume of aluminum, internal compressive stresses are created in the longitudinal direction of the section, which on removing the component from the mold results in corresponding spring-back in the longitudinal direction. By providing lengths of section with local undersizing, the material is made to stretch in the peripheral direction of the section at these places during the HIPF process. Due to the above mentioned plastic constant volume of aluminum, tensile stresses are induced in the longitudinal direction of the section, which on removing the component from the mold, results in corresponding spring-back in the longitudinal direction.
A suitable distribution of stretching and compressive zones minimizes the resultant overall spring-back, so that after the HIPF process the components obtained are accurate in shape.
In order to reshape sharp corners at the same time avoiding excessive local degrees of deformation at the corners, the following measures are taken:
pronounced thickening of the section corners prevents irreversible bending at the start of the HIPF process;
by providing dome-like curvature in the cross-section in the immediate vicinity of the thickened section corners it is possible to reduce, even completely eliminate, the local stretching of the material necessary to reshape small corner radii.
Within the scope of the invention is a hollow section featuring a space delimited by section walls where two section walls define each corner region of the section cross-section and at least one of the section walls adjacent to the comer region exhibits, as viewed in cross-section, a curved region. Preferred is a polygonal cross-sectionxe2x80x94in particular a triangle-shaped cross-sectionxe2x80x94the section walls of which exhibit an inward curved region between each of the comer regions; it is however also possible e.g. to provide only one single wall with a curved region. Usefully, the curved region of section wall should join up with corner regions at both ends. The cross-sectional shape of that curved region may be in the form of part of a circle or part of an ellipse, parabola shaped, hyperbola-like or have some other contour form.
It has been found favorable for such a bent region to exhibit a contour that is in the form of part of a circle, the arc length of which is defined as the distance between a pair of flanges that delimit the related corner regions. That distance is given by the length of section side wall less the lengths of the flanges in the related corner regionxe2x80x94which, depending on the cross-sectional shape of the extrusion and the distribution of wall-thickness may also be unequalxe2x80x94and less the distance defined by the projection of the gap between the initial section and the contour of the shaping tool mold accommodating the component.
Usefully, the length of the flanges in the comer region of the initial section is three to four times the average wall thickness of the lengths of section walls adjacent to the corner region; the length of flange depends on the thickness of the section wall and on the angle these make at the comer region.
In the case of an initial section of cross-section in the form of an equilateral triangle, that distance between the flanges should be e.g. about three times the length of the flange. In this case the height of doming, i.e. the distance between the curvature in the form of part of a circle and a straight line joining the flanges, should correspond approximately to the thick-ness of the section wall.
When using extruded aluminum sections it is possible to avoid the work step involving pre-forming of the sections in that the initial section is manufactured in the desired favorable pre-bent shape. Apart from the savings associated with the pre-forming, at the same time a high degree of process reliability is achieved on bending or on closing the HIPF shaping tool.